1. Field of the Invention
This invention concerns a method and an apparatus for liquefying natural gas. In another aspect, the invention concerns an improved driver, compressor, and power source configurations for a cascade-type natural gas liquefaction plant.
2. Description of the Prior Art
The cryogenic liquefaction of natural gas is routinely practiced as a means of converting natural gas into a more convenient form for transportation and storage. Such liquefaction reduces the volume by about 600-fold and results in a product which can be stored and transported at near atmospheric pressure.
With regard to ease of storage, natural gas is frequently transported by pipeline from the source of supply to a distant market. It is desirable to operate the pipeline under a substantially constant and high load factor but often the deliverability or capacity of the pipeline will exceed demand while at other times the demand may exceed the deliverability of the pipeline. In order to shave off the peaks where demand exceeds supply or the valleys when supply exceeds demand, it is desirable to store the excess gas in such a manner that it can be delivered when the supply exceeds demand. Such practice allows future demand peaks to be met with material from storage. One practical means for doing this is to convert the gas to a liquefied state for storage and to then vaporize the liquid as demand requires.
The liquefaction of natural gas is of even greater importance when transporting gas from a supply source which is separated by great distances from the candidate market and a pipeline either is not available or is impractical. This is particularly true where transport must be made by ocean-going vessels. Ship transportation in the gaseous state is generally not practical because appreciable pressurization is required to significantly reduce the specific volume of the gas. Such pressurization requires the use of more expensive storage containers.
In order to store and transport natural gas in the liquid state, the natural gas is preferably cooled to xe2x88x92240xc2x0 F. to xe2x88x92260xc2x0 F. where the liquefied natural gas (LNG) possesses a near-atmospheric vapor pressure. Numerous systems exist in the prior art for the liquefaction of natural gas in which the gas is liquefied by sequentially passing the gas at an elevated pressure through a plurality of cooling stages whereupon the gas is cooled to successively lower temperatures in sequential refrigeration cycles until the liquefaction temperature is reached. Cooling is generally accomplished by heat exchange with one or more refrigerants such as propane, propylene, ethane, ethylene, methane, nitrogen or combinations of the preceding refrigerants (e.g., mixed refrigerant systems). A liquefaction methodology which is particularly applicable to the current invention employs a closed propane cycle as the initial refrigeration cycle, a closed ethylene cycle as the intermediate refrigerant cycle, and an open methane cycle as the final refrigeration cycle. In the open methane cycle a pressurized LNG-bearing stream is flashed and the flash vapors (i.e., the flash gas stream(s)) are subsequently employed as cooling agents, recompressed, cooled, combined with the processed natural gas feed stream and liquefied thereby producing the pressurized LNG-bearing stream.
Each of the refrigeration cycles of a cascade-type natural gas liquefaction plant includes a compressor, or a set of compressors, for increasing the pressure of the refrigerant after it has been used to cool the natural gas. The high pressure refrigerant exiting the compressor(s) is first cooled via indirect heat exchange and then expanded prior to being employed as a cooling agent to cool the natural gas stream. The refrigerant compressors employed in LNG plants are typically powered by large gas turbines such as, for example, Frame 5 or Frame 7 gas turbines that are available from GE Power Systems of Atlanta, Ga.
Although conventional gas turbines provide efficient power production, the use of gas turbine drivers in LNG plants has several drawbacks. For example, xe2x80x9coff-the-shelfxe2x80x9d gas turbines are available only in predetermined fixed sizes (i.e., load ratings) and it is generally too expensive to have a gas turbine custom designed and manufactured for a certain load requirement. Thus, in many instances commercially available gas turbines are either oversized or undersized for the given application in a LNG plant. This mismatching of optimum design load and actual plant load can require oversized gas turbines to be employed in a LNG plant. Such oversized gas turbines are typically more expensive than would be required if the actual plant load and designed turbine load were the same. Further, operating an oversized gas turbine at less than optimum design load causes the gas turbine to be less efficient.
Another disadvantage of employing gas turbine drivers to power the refrigerant compressors in a LNG plant is that the burning of fuel in the gas turbines causes emissions (e.g., NOx and SO2) that must be monitored in order to comply with local environmental standards. With the increasing stringency of emissions regulations, it can be difficult and expensive to monitor and comply with such regulations.
A further disadvantage of using gas turbines in LNG plants is the fact that only a handful of companies make suitable gas turbines. Thus, availability of an appropriately sized turbine can be severely limited if the demand for that particular turbine is high.
Another drawback of using gas turbines to power compressors in a LNG plant is that gas turbines can be difficult and time consuming to start up.
It is, therefore, an object of the present invention to provide a novel natural gas liquefaction system employing mechanical drivers that can be cost-effectively tailored to suit specific load requirements of the LNG plant.
A further object of the invention is to provide a novel natural gas liquefaction system having reduced emissions due to the use of low-emissions mechanical drivers.
Another object of the invention is to provide a novel natural gas liquefaction system employing mechanical drivers that are readily available from multiple sources throughout the world.
Still another object of the invention is to provide a novel natural gas liquefaction system employing mechanical drivers that are easy and quick to start.
It should be noted that the above objects are exemplary and need not all be accomplished by the claimed invention. Other objects and advantages of the invention will be apparent from the written description and drawings.
Accordingly, in one embodiment of the present invention, there is provided a process for liquefying natural gas comprising the steps of: (a) driving a first compressor and a second compressor with a first electric motor; (b) driving a third compressor and a fourth compressor with a second electric motor; (c) compressing a first refrigerant of a first refrigeration cycle in the first and third compressors; and (d) compressing a second refrigerant of a second refrigeration cycle in the second and fourth compressors.
In another embodiment of the present invention, there is provided a process for liquefying natural gas comprising the steps of: (a) generating steam and electricity in a cogeneration plant; (b) using at least a portion of the electricity to power a first electric motor; (c) using at least a portion of the steam to power a first steam turbine; (d) compressing a first refrigerant of a first refrigeration cycle in a first compressor driven by the first electric motor; and (e) compressing a second refrigerant of a second refrigeration cycle in a second compressor driven by the first steam turbine.
In still another embodiment of the present invention, there is provided an apparatus for liquefying natural gas by cooling the natural gas via a plurality of sequential refrigeration cycles. The apparatus comprises first, second, and third refrigeration cycles and first, second, and third electric motors. The first, second, and third refrigeration cycles include first, second, and third compressors for compressing first, second, and third refrigerants respectively. The first, second, and third electric motors are operable to drive the first, second, and third compressors respectively. The first refrigerant comprises in major portion a hydrocarbon selected from the group consisting of propane, propylene, and mixtures thereof. The second refrigerant comprises in major portion a hydrocarbon selected from the group consisting of ethane, ethylene, and mixtures thereof. The third refrigerant comprises in major portion methane.
In a still further embodiment of the present invention, there is provided an apparatus for liquefying natural gas by cooling the natural gas via a plurality of sequential refrigeration cycles. The apparatus comprises a first refrigeration cycle, a second refrigeration cycle, a cogeneration plant, a first electric motor, and a first steam turbine. The first refrigeration cycle includes a first compressor for compressing a first refrigerant. The second refrigeration cycle includes a second compressor for compressing a second refrigerant. The cogeneration plant is operable to simultaneously generate electricity and steam. The first electric motor is drivably coupled to the first compressor and is powered by at least a portion of the electricity generated by the cogeneration plant. The first steam turbine is drivingly coupled to the second compressor and is powered by at least a portion of the steam generated by the cogeneration plant.